Systematically administered opioids activate bulbospinal noradrenergic pathways. The resulting spinally released norepinephrine activates a2-adrenoceptors on cholinergic neurons, causing ACh release and inhibition of spinal transmission of noxious sensory information. This study tests a computer-based methodology for administering clonidine epidurally to potentiate analgesia from intraspinal a-adrenergic agonist and from systemic opioids.